Leadframe including frame-cutting slit for lead-on-chip (LOC) semiconductor device and semiconductor device incorporating the leadframe

ABSTRACT

A high-reliability semiconductor device and a method of producing the device. Absorption of moisture into a semiconductor device is effectively avoided by using a hard solder material, such as common solder having no moisture absorption, for die-bonding a semiconductor chip to a die pad. Thus, in the semiconductor device in accordance with the present inventions there are no corrosion problems nor package-cracking due to absorbed moisture.

This disclosure is a division of patent application Ser. No. 08/325,637,filed Oct. 19, 1994 U.S. Pat. No. 5,535,509, which is a divisionalpatent application of prior patent application Ser. No. 08/070,990,filed Jun. 4, 1993, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device having alead-on-chip structure with leads extending over a surface of asemiconductor chip, and relates to a method for producing it. Thepresent invention also relates to a leadframe for use in such a deviceor in production of such a device.

2. Description of the Related Art

FIG. 35 is a cross sectional view of a conventional semiconductor devicehaving a lead-on-chip structure (hereafter referred to asLOC-structure). Such a device is disclosed, for example, in JapanesePatent Application Laid-Open No.2-45969. As shown in FIG. 35, the devicecomprises a die pad 1, a semiconductor chip 2, and a plurality of leads3 extending toward the semiconductor chip from its both sides, each ofthe plurality of leads 3 comprising an inner lead portion 3a and anouter lead portion 3b. The device further comprises thin metal wires 5,molding resin 6, and a plurality of electrodes 4 along both sides of aprimary surface of the semiconductor chip 2.

An integrated circuit (not shown) and electrodes 4 are formed byphotolithography or the like on the upper primary surface of thesemiconductor chip 2. The semiconductor chip 2 is fixed on the die pad 1by bonding the back primary surface of the semiconductor chip 2 to thedie pad 1 with a conductive adhesive such as a conductive resin. Theinner lead portions 3a are each electrically connected to the electrodes4 on the semiconductor chip 2 via the thin metal wires 5. These elementsdescribed above are sealed with molding resin 6 except for the outerlead portions 3b so that each outer lead portion 3b of leads 3 isexposed to the outside. The outer lead portions 3b of the leads 3 areeach formed into a desired shape such as a straight type, gull wingtype, J-type, etc.

Japanese Patent Application Laid-Open No.2-45969 also discloses a methodfor producing such a device described above. In accordance with thismethod, two frames are used: a first frame comprising an outer frame anda die pad disposed inside the outer frame, the die pad being connectedto the outer frame via a suspending lead; and a second frame comprisingan outer frame and a plurality of leads extending inward from the outerframe. The die pad of the first frame is sunk (depressed) downward by anamount larger than the thickness of the semiconductor chip. After asemiconductor chip is die-bonded on the die pad of the first frame, thesecond frame is connected to the first frame so that each lead extendsover the semiconductor chip with a predetermined constant spacingbetween each lead and the upper surface of the chip. Then, wire-bondingand resin-molding are performed. The unnecessary portions of frames suchas outer frames are removed to obtain a separate semiconductor device.Finally, for each outer lead portion of the leads is bent, thus acompleted semiconductor device is obtained.

In a conventional semiconductor device with the LOC-structure describedabove, a semiconductor chip is die-bonded onto a die pad with aconductive resin or the like. However, resin materials exhibit moistureabsorption, thus moisture absorbed in resin may cause degradation in theadhesive strength, and/or may corrode the semiconductor chip or leads incontact with the resin. To mount a semiconductor device on a circuitboard or the like, the semiconductor device is put on the circuit board,then the circuit board with the chip are placed in a hot environment toheat it. Thus, the outer lead portions are soldered to the circuitboard. The moisture absorbed in the die-bonding resin confined in themolding resin is evaporated during the soldering process, which mayresult in separation of the semiconductor chip from the die pad, and/orresult in cracks in a package.

In the conventional production method using two frames described above,the outer parts of two frames remain connected to reach other until themolding process is completed. As a result, these outer frames act asobstacles, and difficulty occurs in handling. Therefore, during thesucceeding processes after the two frames are connected, failures oftenoccur in transferring frames, and/or liquid used for processingpenetrates between the outer frames, leaking later to causecontamination. In particular, in the case of a production processincluding a step for plating the outer lead portions of thesemiconductor device before separating the semiconductor device from theframes, serious problems occur because plating solution can penetratebetween two outer frames and may leak some time later.

In another method known in the art, a single frame is used to produce asemiconductor device having LOC-structure, the single frame comprising adie pad and leads formed in an integral fashion. In such a type ofleadframe, a die pad is disposed between the leads extending from bothsides of the leadframe, and the die pad extends in the directionsperpendicular to these leads. However, the width of the die pad cannotbe enlarged exceed the lead area. As a result, only narrow die pads areavailable. Japanese Patent Application Laid-Open No.64-69041 discloses aleadframe having a die pad with a large width extending beyond the areaof the leads. In this case, however, the length of leads, in turn,should be shortened, or otherwise, the leads may be deformed. As aresult, longer thin metal wires are required for wire-bonding and/or thechoice of locations of the electrodes on the semiconductor chip islimited.

In the case where a semiconductor device is produced using a leadframehaving a die pad as well as leads formed in an integral fashion, the diepad is sunk by the amount corresponding to the thickness of asemiconductor chip, and the semiconductor chip is inserted between theleads and the die pad, then the semiconductor chip is die-bonded on thedie pad. However, in the case where the die pad extends perpendicular tothe direction in which the leadframe is transferred during theproduction processes of the semiconductor device, the semiconductor chipmust be inserted between the leads and the sunk die pad in the samedirection as that of the leadframe-transfer path. This insertion processis difficult to perform, and complicated and troublesome operations arerequired. Furthermore, it also requires complicated and troublesomeoperations to die-bond a semiconductor chip onto the die pad with hardsolder after the semiconductor chip is inserted. There are such problemsin conventional semiconductor devices and methods for producing them.

SUMMARY OF THE INVENTION

It is an object of the present invention to solve the problems describedabove. More specifically, it is an object of the present invention toprovide a higher-reliability semiconductor device having a LOC-structurein which no corrosion occurs. It is another object of the presentinvention to provide a method for producing such a semiconductor devicewithout a liquid-leakage problem during production processes. It is afurther object of the present invention to provide a leadframe for usein such a semiconductor device.

To achieve the above objects, in accordance with a first aspect of thepresent invention, a semiconductor device having a LOC-structure isprovided wherein a semiconductor chip is die-bonded onto a die pad witha hard solder material exhibiting no moisture absorption.

In accordance with a second aspect of the present invention, a method isprovided for producing such a semiconductor device using two frames,that is, a frame for the leads and a frame for the die pad, whereinimmediately after these two frames are connected to each other, an outerframe portion of the frame for the die pad is cut off and removed viaframe-cutting slits formed in the frame for leads.

A third aspect of the present invention provides a leadframe comprisingtwo frames including a frame for leads, having frame-cutting slits, foruse in production of a semiconductor device in accordance with thesecond aspect of the invention.

Furthermore, a fourth aspect of the present invention provides aleadframe for use in production of a semiconductor device comprising adie pad and leads formed in an integral fashion, the die pad including amain pad disposed between two sets of leads, one of the sets of leadsbeing formed on one side of an outer frame of the leadframe, and theother set of leads being formed on the other side of the outer frame ofthe leadframe, the main pad extending perpendicular to these leads theleads extending inward from both sides of the outer frame of theleadframe, the die pad further including at least one branch padextending approximately perpendicular to the main pad and extendingbetween two neighboring leads extending from the above outer frame.

A fifth aspect of the present invention provides a method for producinga semiconductor device by using a leadframe in accordance with thefourth aspect of the present invention. In this method, the width of adie pad is widened so as to ensure that a semiconductor chip can belocated on the die pad in a more stable manner. Another feature of thismethod is that when a semiconductor chip is heated via a die pad so asto make wire-bonding easier, the efficiency of heat conduction can beimproved.

A sixth aspect of the present invention provides a leadframe including adie pad extending inward from an outer frame of the leadframe, theleadframe further including leads extending toward the die pad from bothsides of the outer frame of the leadframe, the die pad and the leadsbeing formed in an integral fashion. In this leadframe, the die pad isconnected to the portions of the outer frame on both sides, wherein boththe portions are bent in the same direction perpendicular to theleadframe plane so as to form U-shaped portions, so that the die pad issunk from the outer frame and also from the leads. The U-shaped portionshave a size large enough to insert a semiconductor chip through one ofthese U-shaped portions.

A seventh aspect of the present invention provides a method forproducing a semiconductor device by using a leadframe in accordance withthe sixth aspect of the present invention, the leadframe including a diepad extending in a direction perpendicular to the leadframe-transferdirection, the leadframe further including leads extending toward thedie pad from both sides of the outer frame of the leadframe, the die padand the leads being formed in an integral fashion. In this method,portions of both sides of the outer frame at which the die pad isconnected to the outer frame are bent in the same directionperpendicular to the frame plane so as to form U-shaped portions havinga size large enough to insert a semiconductor chip through one of theseU-shaped portions. Then, a semiconductor chip is inserted through one ofthe U-shaped portions formed in both sides of the outer frame, theU-shaped portions being located at the sides of the leadframe-transferpath.

An eighth aspect of the present invention provides a leadframe includinga die pad and leads formed in an integral fashion within an outer frame,wherein only one end portion of the die pad is connected to the outerframe so that the die pad can be bent outward from the outer frame, andso that a semiconductor chip can be die-bonded onto the die pad bentoutward.

A ninth aspect of the present invention provides a method for producinga semiconductor device by using a leadframe in accordance with theeighth aspect of the present invention. In this method, after bending adie pad outward from an outer frame, a semiconductor chip is die-bondedonto the die pad bent outward. Then, the die pad with the semiconductorchip on it is bent back to the position at which the leads extendingover the semiconductor chip have a predetermined constant space betweenthe leads and the primary surface of the semiconductor chip.

A tenth aspect of the present invention provides a leadframe including adie pad and leads formed in an integral fashion in such a way that thedie pad is formed in a region of the frame and the leads are formed inanother region of the frame adjacent to the first region of the frame.The die pad is connected to the frame via suspending leads. One of thesesuspending leads is cut, and the other suspending lead is bent so thatthe die pad is put over the leads.

An eleventh aspect of the present invention provides a method forproducing a semiconductor device using a leadframe in accordance withthe tenth aspect of the present invention. In this method, afterdie-bonding a semiconductor chip onto a die pad, unnecessary portions ofa frame around the die pad are cut off, then a suspending lead extendingtoward the lead area is bent into a U-shape so that the die pad with thesemiconductor chip is put over the lead area with a predeterminedconstant space between the die pad and the extending leads.

A twelfth aspect of the present invention provides a leadframe modifiedfrom a leadframe of the tenth aspect of the present invention in such away that the suspending lead to be cut is made longer. In a process,this suspending lead is cut in such a way that the suspending leadremains connected to the die pad. After the die pad is folded over theleads, the long suspending lead remaining connected to the die pad isbent toward the leads so as to make a connection between the end portionof this remaining suspending lead and the frame of the lead area. Thus,a firmer connection of the die pad can be achieved.

A thirteenth aspect of the present invention provides a method forproducing a semiconductor device by using a leadframe in accordance withthe twelfth aspect of the present invention. In accordance with thismethod, as described above, a die pad can be fixed more firmly to aframe of a lead area.

A fourteenth aspect of the present invention provides a die-bondingmethod comprising steps of: putting and pressing metal foil on a diepad; putting a semiconductor chip on the metal foil; heating these fromthe surroundings so as to melt the metal foil; stopping heating so as tosolidify the melted metal foil so that the semiconductor chip is fixedto the die pad.

In the first aspect of the present invention, as described above, hardsolder exhibiting no moisture absorption is used as a die-bondingmaterial for die-bonding a semiconductor chip. As a result, moisture isnot absorbed into the die-bonding material. Thus, it is possible toavoid corrosion of the semiconductor chip, and to avoid cracks in apackage.

In the method for producing a semiconductor device in accordance withthe second aspect of the present invention, and in the leadframe for usein the production of the semiconductor device in accordance with thethird aspect of the present invention, after two frames are connected toeach other, unnecessary portions of the frame for the die pad are is cutoff and removed via frame-cutting slits. As a result, during succeedingprocesses, the leadframe can be treated as if it comprises only oneframe. As a result, production processes can be simplified and can beperformed easily. In particular, leakage of plating solution can beeffectively avoided.

In the leadframe and the method for producing a semiconductor deviceusing this leadframe, in accordance with the fourth and fifth aspects ofthe present invention, respectively, the leadframe comprises a die padand leads formed in an integral fashion, the die pad including a mainpad and branch pads extending approximately perpendicular to the mainpad, wherein the width and the area of the die pad are made larger. Eachof the branch pads extends between two adjacent leads among a pluralityof the leads extending inward from an outer frames, thus the existenceof the branch pads causes no limitation in length and shape of theleads. Furthermore, because the width and the area of the die pad areenlarged, a semiconductor chip can be located on the die pad in a morestable manner. Moreover, improvement of the efficiency of heatconduction between the die pad and the semiconductor chip can beachieved and wire-bonding can be performed more easily.

In the sixth and seventh aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe, respectively, a leadframe is used which includes a die padextending in the direction perpendicular to the direction in which theleadframe is transferred during the processes for producing asemiconductor device. A semiconductor chip can be inserted between thedie pad and the leads passing through one of U-shaped portions formed onboth sides of the outer frame, the U-shaped portions being located onthe sides of leadframe-transfer path. Thus, the chip-insertion processis simplified and can be performed easily. In addition, in a leadframein accordance with the sixth aspect of the present invention,independently of the direction in which the die pad extends, the die padcan be sunk from the leads by bending the portions of both sides of theouter frame, at which the die pad is connected to the outer frames, intoa U-shape. This makes the leads extending from one side closer to theleads extending from the other side. Thus, it is possible to make theleads reach the locations closer to the central part of thesemiconductor chip.

In the eighth and ninth aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe wherein the leadframe includes a die pad and leads formed inan integral fashion, the die pad can be bent outward from the outerframe. In this situation in which the die pad is bent outward, asemiconductor chip can be die-bonded onto this die pad. This allows asimpler and easier die-bonding process, compared to the case where asemiconductor chip is inserted between a die pad and leads.

In the tenth and eleventh aspects of the present invention, regarding aleadframe and a method for producing a semiconductor device using thisleadframe wherein the leadframe includes a die pad and leads formed inan integral fashion, a semiconductor chip can be easily die-bonded tothe die pad without any additional special process.

In the twelfth and thirteenth aspects of the present invention,regarding a leadframe and a method for producing a semiconductor deviceusing this leadframe, the die pad can be connected to a frame for leadsmore firmly by modifying the above leadframe and method in accordancewith the tenth and eleventh aspects of the invention in such a way thata suspending lead portion remaining connected to the die pad is bent andis further connected to the frame for leads. Thus, the succeedingprocesses after that can be performed more easily and more accurately.

In the die-bonding method in accordance with the fourteenth aspect ofthe present invention, a metal foil such as solder disposed between adie pad and a semiconductor chip is heated from the surroundings so asto melt the metal foil. Thus, this method allows easier die-bondingprocess, and it provides a significant advantage in particular for thecase where die-bonding is performed after a semiconductor chip isinserted between the die pad and inner lead portions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a plan view of one embodiment of a semiconductor device inaccordance with a first aspect of the present invention;

FIG. 1B is a side view, partially cross sectioned, of the semiconductordevice of FIG. 1A;

FIG. 2 is a perspective view showing the structure of a leadframerelating to second and third aspects of the present invention;

FIGS. 3A-3C are plan views of a semiconductor device for explanation ofone embodiment of successive production steps in accordance with asecond aspect of the present invention;

FIGS. 4A-4D are plan views of a semiconductor device for explanation offurther successive production steps following the step in FIG. 3C;

FIG. 5 is a flowchart showing a method for producing a semiconductordevice according to FIGS. 3A-3C and FIGS. 4A-4D;

FIG. 6 is a side view for explanation of a first embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIG. 7 a side view for explanation of a second embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIG. 8 is a side view for explanation of a third embodiment of a methodfor connecting two frames in accordance with the second aspect of thepresent invention;

FIGS. 9A and 9B are perspective views for explanation of a fourthembodiment of a method for connecting two frames in accordance with thesecond aspect of the present invention;

FIGS. 10A-10D are perspective views for explanation of a fifthembodiment of a method for connecting two frames in accordance with thesecond aspect of the present invention;

FIG. 11 is a plan view of two frames for use in the second aspect of thepresent invention, the two frames being connected to each other;

FIGS. 12A and 12B are side views for explanation of one example ofmethods for cutting a frame with regard to a production method inaccordance with the second aspect of the present invention;

FIG. 13 is a plan view showing one embodiment of a leadframe relating toa fourth and fifth aspects of the present invention;

FIG. 14 is a side view of the leadframe of FIG. 13, seen in thedirection of the arrow XIV of FIG. 13;

FIG. 15 is a cross sectional view showing one embodiment of asemiconductor device produced by a production method in accordance withthe fifth aspect of the present invention;

FIG. 16 is a cross section of the semiconductor device of FIG. 15, takenalong the lines XVI--XVI of FIG. 15;

FIG. 17 is a plan view showing another embodiment of a leadframerelating to the fourth and fifth aspects of the present invention;

FIG. 18 is a perspective view showing another embodiment of a leadframein accordance with the fourth aspect of the present invention;

FIG. 19 is a plan view showing one embodiment of a leadframe relating tosixth and seventh aspects of the present invention;

FIG. 20 is a flowchart showing a method for producing a semiconductordevice in accordance with the seventh aspect of the present invention;

FIG. 21 is a perspective view showing one embodiment of a leadframerelating to eighth and ninth aspects of the present invention;

FIG. 22 is a perspective view showing a die-pad bending step in a methodfor producing a semiconductor device in accordance with the ninth aspectof the present invention;

FIG. 23 is a perspective view showing a die-bonding step in a method forproducing a semiconductor device in accordance with the ninth aspect ofthe present invention;

FIG. 24 is a side view showing a die-pad bending-back step and awire-bonding step in a method for producing a semiconductor device inaccordance with the ninth aspect of the present invention;

FIG. 25 is a perspective view showing another example of a die-padbending step in a method for producing a semiconductor device inaccordance with the ninth aspect of the present invention;

FIG. 26 is a perspective view showing one embodiment of a leadframerelating to tenth and eleventh aspects of the present invention;

FIG. 27 is a perspective view showing a die-bonding step in a method forproducing a semiconductor device in accordance with the eleventh aspectof the present invention;

FIG. 28 is a side view showing a die-pad folding step and a wire-bondingstep in a method for producing a semiconductor device in accordance withthe eleventh aspect of the present invention;

FIG. 29 is a perspective view showing a die-bonding step in a method forproducing a semiconductor device relating to twelfth and thirteenthaspects of the present invention;

FIG. 30 is a side view showing a die-pad folding step, die-pad fixingstep, and wire-bonding step in a method for producing a semiconductordevice in accordance with the thirteenth aspect of the presentinvention;

FIG. 31 is a schematic view showing one embodiment of a die-bonding stepin a method for producing a semiconductor device in accordance with afourteenth aspect of the present invention;

FIG. 32 is a flowchart showing one example of method for producing asemiconductor device in accordance with the ninth aspect of the presentinvention;

FIG. 33 a flowchart showing examples of method for producing asemiconductor device in accordance with the eleventh and thirteenthaspects of the present invention;

FIG. 34 is a flowchart showing one example of a die-bonding methods inaccordance with the fourteenth aspect of the present invention;

FIG. 35 is a cross sectional view of a conventional semiconductor deviceof a similar type.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, referring to the accompanying drawings, the preferred embodimentswill be described below.

The First Aspect of the Invention

FIGS. 1A and 1B show an embodiment of a semiconductor device having aLOC-structure in accordance with the first aspect of the presentinvention, wherein FIG. 1A is a plan view showing inner structure andFIG. 1B is a side view, partially cross sectioned, of the device. Asshown in these figures, the device comprises a die pad 1, asemiconductor chip 2, leads 3 having an inner lead portion 3a and anouter lead portion 3b, electrodes 4, thin metal wires 5, and moldingresin 6 forming a package. This device has substantially the samestructure as that of conventional semiconductor devices except that theelectrodes 4 located along a straight line in a central part of thesemiconductor chip.

There are also provided common leads 3c primarily used for supplyingelectrical power or used as a ground lead, wherein the common leads 3cextend over the central portion of the semiconductor chip 2 in thelongitudinal direction. In addition, hard solder 7 such as conventionalsolder is also provided for die-bonding the semiconductor chip 2 to thedie pad 1. The hard solder 7 exhibits no moisture absorption, thus thereis no possibility that moisture will corrode the semiconductor chip 2.Furthermore, there is no possibility that moisture brings about cracksin a package, even if the whole device is heated when a completedsemiconductor device is attached to a circuit board (not shown) bysoldering the outer lead portions 3b. Because the hard solder material 7for connecting the semiconductor chip 2 to the die pad is disposed in aninner portion covered with molding resin 6, less heat is provided tothis hard solder material 7 than to the external portions. Thus, thehard solder material 7 does not melt during the soldering process. Ifnecessary, the hard solder material 7 may be selected so that it has amelting point high enough not to be melted during soldering process formounting the semiconductor device on a circuit board. In the case ofconventional solder, this result can be achieved by selecting themixture ratio between tin and lead.

The Second and Third Aspects of the Invention

FIG. 2 shows a leadframe comprising two frames in accordance with thesecond and third aspects of the present invention. The second aspect ofthe present invention relates to a method for producing a semiconductorchip using a leadframe, and the third one relates to the leadframeitself. As shown in FIG. 2, a leadframe 80 comprises a frame 81 forleads and a frame 82 for a die pad for use in the present invention.These frames 81 and 82 are each produced from one metal sheet by meansof punching or etching. The frame 81 for leads comprises common leads 3cand a plurality of leads 3 extending inward from both sides of the outerframe 81a. Each of the leads 3 comprises an inner lead portion 3a and anouter lead portion 3b. The frame 81 for leads has frame-cutting slits 18formed in the outer frame 81a. The frame-cutting slits 18 will bedescribed later in more detail. The frame 82 for die pad includes a diepad 1, both sides of the die pad 1 being connected to inner portions ofan outer frame 82a via suspending leads 82b. The die pad 1 is formed ina shape similar to the shape of a semiconductor chip 2 to be die-bondedso that the semiconductor chip 2 can be fixed onto the die pad in a morestable manner. The semiconductor chip 2 is die-bonded to the die pad 1via a hard solder material 7, as shown in FIG. 2. In the frame 82 for adie pad, the suspending leads on the both sides are bent by an amountdetermined by taking the thickness of the semiconductor chip 2 intoaccount, so that the die pad 1 is sunk from the surrounding outer frame82a. Thus, when the frames 81 and 82 are connected to each other, thecommon leads 3c and the inner lead portion 3a of each lead 3 extend overand parallel to the primary surface of the semiconductor chip 2, with apredetermined constant spacing between the leads and the surface of thesemiconductor chip 2. The method of connection between the frame 81 forleads and the frame 82 for a die pad will be described later.

In known technologies, a die pad is formed in an integral fashionbetween common leads 3c or between inner lead portions 3a extending fromthe both sides of an outer frame 81a of a frame 81 for leads. However,in such a frame, the width of the die pad 1 cannot be widened beyond theregion of the leads. In contrast, in this specific aspect of the presentinvention, wherein two frames, that is, a frame 81 for leads and a frame82 for a die pad, are connected to each other, the shape of die pad 1can be designed independently of the shape of leads, thus the width ofthe die pad 1 can be widened, as required. A semiconductor chip 2 may bealso die-bonded in a different way such that after two frames 81 and 82are connected to each other, the semiconductor chip 2 is between the diepad 1 and the inner lead portions 3a, and fixed to the die pad 1.

A method for producing a semiconductor device in accordance with thesecond aspect of the present invention is illustrated in FIGS. 3A-3C andFIGS. 4A-4D. A flowchart of this production method is also shown in FIG.5. Now, the successive steps of this method will be described next.First, a semiconductor chip 2 is fixed to a die pad 1 of a frame 82 fordie pad shown in FIG. 3A, using a hard solder material 7 such asconventional solder exhibiting no moisture absorption (See FIGS. 1 and2). Now, the situation becomes as shown in FIG. 3B (die-bonding stepS1). Then, a frame 81 for leads having frame-cutting slits 18 as shownin FIG. 3C is put on the frame 82 for a die pad which is the situationas shown in FIG. 3B, and the frames are connected to each other as shownin FIG. 4A, by one of methods (for example, spot welding) which will bedescribed later (connecting step S2).

Then, an unnecessary outer frame 82a of the frame 82 for a die pad iscut off by means of die-cutting. Thus, the die pad 1 with thesemiconductor chip 2 remains connected to connecting portions 17 on bothsides (outer-frame cutting-off step S3). After that, the resultantcombined frames can be treated as a leadframe, and the successiveproduction processes can be performed easily. Moreover, this results inelimination of the possibility of accumulation or leakage of platingsolution used in a later process of exterior-plating. Then, electrodes 4(see FIG. 1) located along the center line of the semiconductor chip 2are electrically connected via the thin metal wires 5 to the inner leadportions 3a and/or the common leads 3c extending over the chip 2(wire-bonding step S4).

After wire-bonding is performed, the frame is put into a mold (notshown), then resin is injected into the mold to seal the main padincluding the semiconductor chip 2 with molding resin 6 as shown in FIG.4D (molding step S5). In this step, molding is performed in such a waythat the outer lead portions 3b are not covered with the resin but areexposed to the outside. The following steps are not shown in figures,but the outer lead portions 3b are exterior-plated (exterior-platingstep S6), then the semiconductor device including the outer leadportions 3b are cut off from the frame and tie-bars are also cut off soas to separate the outer lead portions 3b into individual leads.Furthermore, the separated outer lead portions 3b are formed into adesired shape in lead forming step S7, so that a completed semiconductordevice is obtained.

The frame-cutting slits 18 may be formed in any one of two frames. Theshape of the slits is not limited to that shown in figures. Instead ofdie-cutting, laser cutting or the like may be used for the same purposefor cutting off the outer-frame portion of the frame 82 for die pad. Inthis specific example in accordance with the first aspect of the presentinvention, the semiconductor chip 2 is die-bonded with a hard soldermaterial 7, however other materials may be used not only in this firstaspect but also in any of the other aspects of the present invention.

Now, in accordance with the second aspect of the present invention,there will be described some examples of the methods for connecting aframe 81 for leads to a frame 82 for a die pad. FIG. 6 shows a firstembodiment of a method for connecting a frame 81 for leads to a frame 82for a die pad. In this embodiment shown in FIG. 6, connection isperformed by means of resistance welding which is one of thespot-welding techniques. In this figure, there are shown weldingelectrodes 9, an electrode-holding bar 10a for holding the upper weldingelectrode 9, and an electrode-holding base 10b for holding the lowerwelding electrode 9. A frame 81 for leads and a frame 82 for a die padare put between the upper and lower welding electrodes and pressed bythem by moving the electrode-holding bar 10a in the direction denoted bythe arrow A in FIG. 6. By supplying a current between the upper andlower welding electrodes 9, the frames 81 and 82 are spot-welded.Resistance welding has the advantages that large bonding strength can beobtained and that only little deformation occurs at the surface of awelded portion. Another great advantage of resistance welding is thatwelding dust is not generated, thus extremely little contaminationoccurs to the semiconductor chip 2. Spot welding can be also performedby means of laser welding.

FIG. 7 shows a second embodiment of a method for connecting a frame 81for leads to a frame 82 for a die pad. In this embodiment shown in FIG.7, connection is performed by using an adhesive tape. In this figure,there are shown an adhesive tape 11, a pressing fixture 12, and aframe-holding base 13. An adhesive tape 11 is placed between frame 81for leads and a frame 82 for a die pad, and these frames 81 and 82 withthe adhesive tape 11 are put on the frame-holding base 13 and arepressed by the pressing fixture 12 so that both frames 81 and 82 areconnected to each other via the adhesive tape 11. This method has anadvantage over a laser welding technique that it can offer highthroughput and also has an advantage over a spot welding technique thatthere is no need for maintenance of equipment.

FIG. 8 shows a third embodiment of a method for connecting a frame 81for leads to a frame 82 for a die pad. In this embodiment shown in FIG.8, connection is also performed using an adhesive tape in a similar wayto that of FIG. 7, however, in this case, the adhesive tape 11 comprisesan adhesive tape base 11a and thermoplastic adhesives 11b on both of itssurfaces. In this embodiment, each surface of the adhesive tape 11 isbonded to the frame 81 or frame 82 via an interfacial material ofthermoplastic adhesive 11b. As a result, even if a thermal history isintroduced during processing steps such as wire-bonding and molding(sealing), the thermoplastic adhesives 11b absorb the stress introducedby the difference in thermal expansion between frames 81 and 82, thusthe total thermal deformation of the frames can be minimized. Althoughan adhesive tape is used in these connecting methods shown in FIGS. 7and 8, the connection is performed outside of the area to be molded withmolding resin 6. Thus, even if the adhesive tape exhibits moistureabsorption, no problem will occur.

FIGS. 9A and 9B show a fourth embodiment of a method for connecting aframe 81 for leads to a frame 82 for a die pad. In this embodiment,connection is performed using a rivet. In these figures, there are shownholes 14 in each of the frames 81 and 82, and a rivet 15. As shown inFIG. 9A, two frames 81 and 82 are put one on top of the other, then arivet 15 is inserted through holes 14. Then, the upper end of the rivet15 is pressed and spread out so as to fasten two fitted frames 81 and82. The shapes of the rivet 15 and holes 14 are not limited to thoseshown in figures. Instead of the rivet 15, any other element havingsimilar function may be used.

FIGS. 10A-10D show a fifth embodiment of a method for connecting a frame81 for leads to a frame 82 for a die pad. In this embodiment, connectionis performed by means of caulking. In these figures, there are shown aprojection 16 on a frame 82, a hole 14 in a frame 81, wherein theprojection 16 is to be fitted into the hole 14. First, a projection isformed on a frame 82 as shown in FIG. 10A by means of etching orpunching. Then, the projection 16 is bent upward as shown in FIG. 10B. Ahole 14 corresponding to the projection 16 is formed in a frame 81 bymeans of etching or punching. One of two frames 81 and 82 is put on theother in such a way that the projection 16 is inserted and fitted in thehole 14. Then, the projection 16 is bent back as shown in FIG. 10D.Thus, the frames 81 and 82 are fitted and fastened by means of calking.The shapes of the projection 16 and the hole 14, and the method forforming them are not limited to those described above.

FIGS. 11, 12A and 12B show one embodiment of a method for cutting off anunnecessary portion of an outer frame part 82a of a frame 82 for a diepad which has been fastened to a frame 81 for leads by means of anymethod described above. FIG. 11 shows a frame 82 for die pad with asemiconductor chip mounted on a die pad, the frame 82 for a die padbeing connected to the bottom of a frame 81 for leads. In this figure,there are shown connecting portions 17 (for example, spot weldingportions) at which the frames are connected to each other, andframe-cutting slits 18 in the frame 81 for leads.

The frame-cutting slits 18 in the frame 81 for leads as described aboveoffer a means for cutting off the frame 82 for die pad except for a diepad 2 (including suspending leads) connected to the frame 81 atconnecting portions 17 on both sides. As a result of the cutting, theframe 81 having the die pad 1 with the semiconductor chip 2 can betreated almost like a single sheet frame. Thus, during the succeedingprocesses, problems such as miss-transfer of frames, and accumulationand leakage of solution which often occur in two combined frames can beeffectively avoided.

This cutting method is shown in FIGS. 12A and 12B where a lower metalfixture is denoted by 19, a frame holder by 20, and a cutting metalelement by 21. As shown in FIG. 12A, the frame 81 for leads and die padframe 82 which are connected to each other at the connecting portion 17are put on the lower metal fixture 19 and held by the frame holder 20.The cutting metal element 21 is inserted into the frame-cutting slits 18down to a position at which the cutting metal element 21 stops with apredetermined adequate gap from the lower metal fixture 19. Thus, theouter frame of the frame 82 for die pad is cut off from the frame 81 forleads.

As described previously, one known method for producing a semiconductordevice having LOC-structure is to use a one-sheet leadframe having leadsand a die pad formed in an integral fashion. Such a frame can beproduced from a sheet of metal by means of cutting such as punching oretching. Although this method has an advantage that fewer processingsteps are required for producing a semiconductor device compared to thecase where two frames are combined, it has disadvantages that because ofits structure it is difficult to expand the width of a die pad exceed alead area, and that a semiconductor chip cannot be die-bonded on a diepad in a stable fashion. Now, to solve such problems, in accordance withthe fourth aspect of the present invention, a leadframe for use in asemiconductor device having a LOC-structure as well as a method inaccordance with the fifth aspect of the present invention for producinga semiconductor device having LOC-structure using such a leadframe willbe described below.

The Fourth and Fifth Aspects of the Invention

FIGS. 13 and 14 show an example of a leadframe in accordance with thefourth and fifth aspects of the present invention, wherein FIG. 13 is atop view and FIG. 14 is a side view seen in the direction denoted by anarrow XIV in FIG. 13. As shown in FIG. 13, this leadframe 83 comprises aplurality of leads 3 inside of an outer frame 83a and also comprises adie pad 100 extending in the direction approximately perpendicular tothe leads 3, the leads 3 and the die pad 100 being unitary. The die pad100 comprises a conventional type of main pad 101 extending along thearea corresponding to a central area of a semiconductor chip 2 in arange of its full length, and further comprises branch pads 102extending from both sides of the main pad 101 in a cross shape. Thus,the width of the die pad is substantially widened. Each branch pad 102is in the same plane 95 the main pad 101, and extends approximatelyperpendicular to the main-pad 101. Furthermore, each branch pad 102extends between neighboring leads among a plurality of leads 3 extendinginward from both sides of the outer frame 83a at a predeterminedinterval. Because of such a structure, the die pad 100 can be widened,exceeding the lead area, without modifying the location, shape, and/orlength of leads 3. As shown in FIG. 14, the die pad 100 is sunk from theouter frame 83a, and a semiconductor chip 2 is die-bonded onto the sunkdie pad 100 with, for example, hard solder.

FIGS. 15 and 16 show a semiconductor device produced by a productionmethod in accordance with the fifth aspect of the present invention,using a leadframe shown in FIG. 13. FIG. 15 is a cross sectional viewtaken along a lead 3 of the semiconductor device. FIG. 16 is a crosssectional view taken in the line XVI-XVI of FIG. 15. The basic structureis the same as that of a semiconductor device in accordance with thefirst aspect of the present invention shown FIG. 1, thus the same orsimilar portions are denoted by the same numerals. Explanation will notbe repeated again for these same or similar portions. In thesemiconductor device shown in FIGS. 15 and 16, there are provided onlyleads 3 extending over the semiconductor chip 2 and there are no commonleads 3. The flowchart showing this production method will be the sameas that in accordance with the second aspect of the present inventionshown in FIG. 5 except that steps S2 and S3 are deleted.

Now, the method for producing a semiconductor device shown in FIGS. 15and 16 will be described. As described above in connection with FIG. 14,a semiconductor chip 2 is inserted between the outer frame 83a of theleadframe 83 and the sunk die pad 100. Then, the semiconductor chip 2 isattached to the die pad 100 with for example, hard solder 7 (die bondingstep S1). As shown in FIG. 15, each of leads 3 floats without contactwith the upper surface of the semiconductor chip 2.

Then, inner lead portions 3a of the leads 3 extending over thesemiconductor chip 2 ate each connected to electrodes 4 on thesemiconductor chip 2 with thin metal wires 5 by means ofultrasonic-thermocompression wire-bonding (wire bonding step S4). Inthis step, when a thin metal wire 5 is compressed onto an electrode 4 onthe semiconductor chip 2, mechanical force is applied to thesemiconductor chip 2. However, due to the good stability of thesemiconductor chip 2 die-bonded to the die pad 100, it is ensured thatwire bonding can be performed easily with no problems. During thewire-bonding process, the die pad is heated so as to raise thetemperature of the semiconductor chip 2 for easier wire bonding. Becausethe die pad 100 has a larger area than a conventional leadframe havingleads and a die pad formed in an integrated fashion, better thermalconduction can be achieved from the die pad 100 to the semiconductorchip 2. Thus, the temperature of the semiconductor chip 2 can be raisedmore effectively.

Then, the semiconductor chip 2, the die pad 100, the inner lead portions3a, and thin metal wires 5 are encapsulated with, for example, epoxyresin into one body by means of transfer molding, thus molding resin 6is completed (molding step S5). Each outer lead portion 3b extendingoutward from the molding resin 6 is exterior-plated (exterior platingstep S6). Finally, an assembled semiconductor device is separated fromthe outer frame 83a of the leadframe 83, then each outer lead portion 3bextending outward from the molding resin 6 of the semiconductor deviceis formed into a desired shape (lead forming step S7). Thus, a completedsemiconductor device is obtained. A leadframe 83 used in actualproduction comprises a plurality of unit elements such one shown in FIG.13, the unit elements being successively connected to each other, and aplurality of semiconductor devices are produced at a time in a similarway to that of the second aspect of the present invention. Ifunnecessary, exterior plating may not be performed, not only in thisspecific production method but in any of production methods which willbe described later.

In the completed semiconductor device as in FIGS. 15 and 16, unlikesemiconductor devices produced by using a conventional one-sheet frame,the back surface of the semiconductor chip 2 has a smaller area indirect contact with the molding resin 6, thus better adhesion isachieved between the back surface of the semiconductor chip 2 and themolding resin 6, and separation is avoided.

In the above specific embodiment, the die pad 100 has a shape of across.However, the branch pad 102 may be formed only on one side of the mainpad 101 of the die pad 100. In contrast to this, in a leadframe 83 shownin FIG. 17, each side of a main pad 101 of a die pad 100 has a pluralityof branch pads 102 extending from each side. Each branch pad 102 of thisleadframe 83 extends between neighboring leads 3. By using such aleadframe 83 having a die pad 100 with a larger number of branch pads102, the stability of the semiconductor chip and thermal conductionefficiency during the wire bonding process step are improved.Furthermore, a completed semiconductor device exhibits better adhesionbetween the back surface of a semiconductor chip and molding resin.

As described above in connection with FIGS. 13 and 17, in a leadframe inaccordance with the fourth aspect of the present invention, a die pad100 extends in the direction perpendicular to the longitudinal directionof a leadframe 83 (actual leadframe comprises a plurality of unitelements shown in these figures, the unit elements being successivelyconnected in the horizontal direction of these figures). Therefore, toinsert a semiconductor chip 2 between a die pad 100 and an outer frame83a, the chip 2 is inserted in the longitudinal direction of theleadframe 83 (the direction denoted by the arrow XIV in FIG. 13). Inproduction lines, in general, leadframes are transferred in thelongitudinal direction of the leadframes. This means that there isdifficulty in chip-insertion process due to the fact that semiconductorchips are inserted in the same direction as that of theleadframe-transfer path in the case of the leadframe 83 shown in FIGS.13 and 17.

In view of the above, another embodiment is provided in accordance withthe fourth aspect of the present invention as is shown in FIG. 18. Inthe case of a leadframe 83 shown in FIG. 18, a main pad 101 of a die pad100 extends in a longitudinal direction of the leadframe 83, that is, inthe same direction as that in which the leadframe 83 is transferred, andeach lead 3 extends perpendicular to this direction. Both end portionsof the die pad 100 are bent such that the die pad 100 is sunk from theouter frame 83a. Thus, a semiconductor chip can be inserted from a sideposition of the leadframe-transfer path, that is, in the directionperpendicular to the leadframe-transfer path, as shown by the arrow B.In this way, an easier chip-insertion process is achieved. In FIG. 18,there are shown tie-bars 3d for connecting leads 3 to each other, whichare not shown in figures for previous embodiments. These tie-bars 3d arecut off and separated into individual outer leads 3b in lead cutting andforming process in which the semiconductor device is also separated fromthe outer frame 83a.

The Sixth and Seventh Aspects of the Invention

In accordance with the sixth and seventh aspects of the presentinvention, FIG. 19 shows a leadframe which is modified from theleadframe shown in FIG. 13 or 17 so that a semiconductor chip can beinserted from a side position of a leadframe-transfer path. The sixthaspect of the invention relates to a leadframe and the seventh to amethod for producing a semiconductor device using a leadframe of thesixth aspect of the invention. In previous embodiments, a die pad 100 issunk from an outer frame 83a by bending both end portions (suspendingleads) of the die pad 100, as shown in FIG. 14. In contrast, in aleadframe 84 shown in FIG. 19, both side portions of an outer frame 84ato which a die pad 100 is connected are bent into U-shape (U-shapedportions 84b) so that the die pad 100 is sunk from the outer frame 83a.Such a leadframe having a die pad 100 sunk by bending portions of theouter frame 84a into U-shape has following advantages: (1) The die pad100 can be sunk more deeply; (2) This technology can be adapted even toa thin leadframe having small mechanical strength; and (3) Leads 3extending from both sides can be located close to the leads on theopposite side. These advantages can be obtained independently of therelation in directions between the die pad and the frame-transfer paths.Thus, a leadframe in accordance with the sixth aspect of the presentinvention has no limitations in on a direction in which a die padextends, and it can be used in various fashions.

FIG. 20 is a flowchart showing a production method in accordance withthe seventh aspect of the present invention.

Now, an example of a production method in accordance with the seventhaspect of the present invention will be described. First, by punching oretching a sheet of flat metal (not shown) in a similar way to those inthe embodiments described above, a leadframe 84 is produced in a planarform which is the situation prior to that shown in FIG. 19 (leadframeproduction step S1). A die pad 100 is formed in such a way that its mainpad 101 extends perpendicular to the direction in which the leadframe 84is to be transferred during production steps of a semiconductor device(that is, perpendicular to the longitudinal direction of a leadframe, inthis case). Then, the portions of both sides of outer frame 84a to whichthe die pad is connected (that is, the portions of outer frame locatedat both sides of leadframe-transfer path) are bent by means of pressingso as to form U-shaped portions 84b. These U-shaped portions areprojected in the same direction, approximately perpendicular to theframe plane. In this way, the die pad 100 is sunk from the outer frame84a and inner lead portions 3a (die-pad depression step S2). Asemiconductor chip (not shown) is inserted in the direction denoted byan arrow B between the die pad 100 and the inner lead portions 3a viaone of these U-shaped portions 84b (semiconductor chip insertion step53). Therefore, the U-shaped portions 84b should have a large enoughsize for a semiconductor chip to be inserted via one of them. After thisstep, a semiconductor device is completed through the steps similar tothose in the case of previous embodiments such as die bonding step S4,wire bonding step S5, molding step S6, exterior-plating step S7, andlead forming step S8.

In accordance with this aspect of the present invention, leadframes 84are transferred in the longitudinal direction of the leadframes 84during production processes and semiconductor chips are insertedtransverse to the transfer path. Thus, the chip-insertion process can beperformed easily. Furthermore, because both sides of the outer frame 84aare bent into the U-shape, it is possible to shorten the distance fromthe inner lead portions 3a extending from one side of the outer frameand those extending from the other side. As a result, the central leadportions 3a can extend to more inner locations of the semiconductorchip.

The Eighth and Ninth Aspects of the Invention

The present invention further includes another method for producing asemiconductor device using a unitary sheet frame comprising a die padand leads.

In accordance with eighth and ninth aspects of the present invention, afirst embodiment is shown in FIGS. 21-24 relating to a leadframe and amethod for producing a semiconductor device using this leadframe. Inthese figures, there are shown a leadframe 85 having an outer frame 85a,a die pad 100, and a plurality of leads 3. The die pad 100 comprises amain pad 101 and branch pads 102. Each lead 3 comprises an inner leadportion 3a and an outer lead portion 3b. There are also providedtie-bars 3d for connecting leads to each other. This leadframe is thesame as that shown in FIG. 13 in accordance with the fourth aspect ofthe present invention except that only one end portion of the die pad100 is connected to the outer frame 85a. FIG. 32 is a flowchart showinga production method in accordance with the ninth aspect of theinvention.

Referring to the figures, one embodiment of a production method inaccordance with the ninth aspect of the invention will be describedbelow. First, for example, one flat sheet of metal (not shown) is cut bypunching or etching so as to produce a leadframe 85 as in FIG. 21. Here,one end of a die pad 100 is connected to an outer frame 85a, and aplurality of leads 3 extend from both sides of the outer frame 85atoward the die pad 100. Then, the die pad 100 is bent outward from theouter frame 85a as shown in FIG. 22 (die pad bending step S1). In thisstep, the die pad 100 is bent such that the die pad 100 is approximatelyperpendicular (at 90 degrees) to the outer frame 85a. Then, as shown inFIG. 23, a semiconductor chip 2 is attached onto the bent die pad 100with hard solder 7 such as common solder (die bonding step S2). In thisway, a semiconductor chip can be easily die-bonded onto a die pad 100.

Now, as shown in FIG. 24, the die pad 100, having the semiconductor chip2 mounted on it, is bent back toward the outer frame 85a (die-padbending-back step S3). As shown, the die pad 100 is bent in the form ofL-shape so that the inner lead portion 3a of each lead 3 extends over aprimary surface of the semiconductor chip 2 having electrodes 4 and acircuit (not shown) on the primary surface of the semiconductor chip 2in such a way that a predetermined constant spacing is maintainedbetween the primary surface and the inner lead portions 3a. Then, therespective electrodes 4 on the semiconductor chip 4 are connected tocorresponding inner lead portions 3a of leads 3 with thin metal wires 5by means of ultrasonic-thermocompression wire bonding, thus electricalconnections are achieved (wire bonding step S4). After this step, asemiconductor device is completed through the steps similar to those inthe case of previous embodiments such as molding step S5,exterior-plating step S6, and lead forming step S7. In the abovespecific embodiment, a semiconductor chip 2 is die-bonded to the die pad100 after the die pad 100 is bent by 90 degrees with respect to theouter frame 85a. However the bending angle of the die pad 100 is not solimited. The die pad 100 may be bent by 180 degrees as shown in FIG. 25,or by any arbitrary angle which results in easy die bonding.

The Tenth and Eleventh Aspects of the Invention

In accordance with tenth and eleventh aspects of the present invention,a first embodiment is shown in FIGS. 26-28 relating to a leadframe and amethod for producing a semiconductor device using this leadframe. Inthese figures, there is shown a wide leadframe 86 comprising an outerframe 86a, a die pad 110 in the sheet form, and leads 3. The outer frame86a includes three strips. The die pad 110 has a size similar to that ofa semiconductor chip 2 to be mounted on it. The ends of the die pad 110are connected respectively to first and second strips of the outer frame86a via suspending leads 111. The leads 3 are formed between the secondand third strips. A production method in accordance with the eleventhaspect of the present invention is shown in a flowchart of FIG. 33.

Referring to the figures, a production method in accordance with theeleventh aspect of the present invention will be described below. First,as shown in FIG. 26, for example one flat sheet of metal (not shown) iscut by punching or etching so as to produce a wide leadframe 86 having adie pad and leads formed in a unitary fashion. In this leadframe 86,leads 3 are formed in a half-unit frame area and a die pad is formed inanother neighboring half-unit frame area. Because the leadframe 86 isformed in such a way described above, the die pad 110 can be formed inthe shape of a sheet which makes it possible to directly bond a chiponto the die pad without any further modification. As shown in FIG. 27,a semiconductor chip 2 is attached onto the die pad 110 with hard solder7 such as common solder (die bonding step S1). Then, the outer frame 86ais cut at three half-etched portions 86b so as to remove an unnecessaryportion (the lower portion including the first strip in FIG. 27) of theouter frame 86a around the die pad 110 (outer-frame cutting step S2).Furthermore, the die pad 110 with the mounted semiconductor chip 2 isbent at the suspending lead 111 into the U-shape so that the die pad 110is folded toward the half-unit frame area of the outer frame 86a inwhich the leads 3 are formed, as shown in a cross sectional view of FIG.28 (die-pad folding step S3). In this way, the inner lead portion 3a ofeach lead 3 is arranged with a predetermined constant spacing from aprimary surface (upper surface) of the semiconductor chip 2 whereinelectrodes 4 and a circuit (not shown) are formed on the primarysurface. Then, a semiconductor device is completed through steps similarto those in the case of previous embodiments, such as a wire bondingstep S4, molding step S5, exterior-plating step S6, and lead formingstep S7. Japanese Patent Application Laid-Open No.63-34966 discloses atechnology in which a leadframe is produced in the form of a sheethaving a die pad and leads formed in an integrated fashion, and an areaof the outer frame of the leadframe is folded so as to put leadsopposite the die pad in a similar way to that of the present aspect ofthe invention. However, there is a significant difference that in thepresent aspect of the invention an unnecessary portion of an outer framearound a die pad is cut off, and then a suspending lead 111 forconnecting the die pad to the outer-frame is bent into the U-shape so asto put the die pad opposite the leads. As can be seen, not only can theabove process steps be easily performed, but also in succeeding steps,the leadframe can be easily handled. Moreover, because the unnecessaryportion is removed, fewer transfer errors occur during the succeedingprocess steps.

The Twelfth and Thirteenth Aspects of the Invention

FIGS. 29 and 30 show a leadframe and a method for producing asemiconductor device using this leadframe in accordance with twelfth andthirteenth aspects of the present invention. The differences from theleadframe described above in connection with FIG. 27 are that one ofsuspending leads on opposite sides of a die pad, that is, a suspendinglead 112 extending in the direction opposite to leads 3, and beingconnected to a first strip is made longer there is also provided athinner half-etched portion 113 at the end of the suspending lead 112,and a fixing means, that is, a positioning hole 86c in a strip locatedopposite the die pad 110 in an area of frame 86a in which the leads 3are formed. The process steps of this embodiment can be described by aflowchart added with a step denoted by dotted lines in FIG. 33, whereinFIG. 33 is a flowchart showing the eleventh aspect of the presentinvention.

In this embodiment, in a process step for cutting off an unnecessaryportion of the outer frame 86a around the die pad 110 as well as thehalf-etched portions 86b of the outer frame 86a, the half-etched portion113 is cut so that the longer suspending lead 112 remains connected tothe die pad 110. Then, by bending a suspending lead 111 into the U-shapeas shown in FIG. 30, the die pad 110 with a mounted semiconductor chip 2is folded toward the frame area 86a in which the leads 3 are formed(die-pad folding step S3). After that, the longer suspending lead 112 isbent into the L-shape and the thinned half-etched portion 113 isinserted into (or connected to) the positioning hole 86c in the strip ofthe outer frame 86a fix the die pad 110 to the leads 3 (die-pad fixingstep S8). Thus, the present aspect of the invention provides a methodadaptable to the mass production, which ensures that the distancebetween the surface of the semiconductor chip 2 and the inner leadportions 3a can be kept at a certain constant value, which furtherresults in better production yields. The other process steps are thesame as those in the case of the eleventh aspect of the presentinvention.

The Fourteenth Aspect of the Invention

In accordance with the fourteenth aspect of the present invention, therewill be described a preferred embodiment of a die-bonding method whichcan be used in the die-bonding steps of the above-mentioned methods forproducing the semiconductor devices. FIG. 31 shows an example ofdie-bonding methods used in production of semiconductor device, inaccordance with the fourteenth aspect of the present invention. FIG. 34is a flowchart showing this die-bonding method. In actual production, asdescribed earlier, in order to produce a plurality of semiconductordevices at one time, a leadframe, for example the leadframe 85 shown inFIGS. 21 and 22, comprises a plurality of unit elements successivelyconnected to neighboring unit elements, wherein each unit elementcomprises a die pad 100 and leads 3. The process steps described beloware successively performed for one unit element at one time by shiftingthe leadframe 85 by one block (one pitch). In the first step, a metalfoil comprising, for example, hard solder 7 such as conventional solderis placed on and pressed onto a die pad 100 (metal foil putting andpressing step S1). Then, the leadframe 85 is shifted by one pitch and asemiconductor chip 2 is put on the metal foil 7 on the die pad 100(semiconductor chip putting step S2). After another one-pitch shifting,the die pad 100 is heated via its bottom portion by non-contactingheating means such as radiation from a heating lamp 50 so as to melt themetal foil 7 (metal foil melting step S3). By stopping heating (or byintentionally cooling), the melted metal foil 7 is solidified and thesemiconductor chip 2 is connected to the die pad 100.

In actual production lines, in accordance with the die-bonding methoddescribed above, die bonding can be easily performed by shifting aleadframe by one pitch at a time. Especially in the case where asemiconductor chip is inserted between leads of a leadframe and a sunkdie pad, this method makes the die-bonding process significantly easier.In the specific embodiment described above, the metal foil 7 is meltedby heating it via the bottom of a die pad so that a circuit formed onthe upper surface of a semiconductor chip is not be damaged. However, ifthe degree of heating is below a permissible level, the heating can beperformed from the upper side or from both upper and lower sides.

As described above, the present invention provides a various advantages.

In a semiconductor device in accordance with the first aspect of thepresent invention, hard solder exhibiting no moisture absorption is usedas a die-bonding material for die-bonding a semiconductor chip. As aresult, moisture is not absorbed into the die-bonding material, and itis possible to avoid problems such as corrosion of the semiconductorchip, cracks in a package, and separation between the semiconductor chipand the die pad. Thus, a higher-reliability semiconductor device isobtained.

In a method in accordance with the second aspect of the presentinvention for producing a semiconductor device, after two leadframes,that is, one for leads and the other for a die pad, are connected toeach other, an unnecessary portion of an outer frame of the leadframefor a die pad is cut off and removed from the other portions viaframe-cutting slits formed in the frame for leads. Thus, during thesucceeding processes after that, the leadframes can be treated as if theleadframe comprises only one frame. As a result, production processescan be simplified and can be performed easily. Therefore, the productionefficiency and production yield can be improved. The third aspect of thepresent invention provides a leadframe having an advantage that aftertwo leadframes, one for leads and the other for a die pad, are connectedto each other, an unnecessary portion of one frame can be cut off andremoved from the other portions.

In accordance with the fourth aspect of the present invention, aleadframe comprises a die pad including branch pads formed on both sidesof the main die-pad portion, wherein each branch pad extendsperpendicular to the main die-pad portion, and wherein each branch padextends between leads. In this way, the width and the area of the diepad can be made substantially larger without modifying leads. The fifthaspect of the present invention provides a production method which makespossible to hold and fix a semiconductor chip on a die pad in a morestable manner, and further to improve thermal conduction efficiencybetween the die pad and the semiconductor chip. As a result, productionprocesses especially a wire-bonding process can be performed easily,accurately, and efficiently. Thus, a high-reliability semiconductordevice can be produced. Compared to a semiconductor device produced byusing a conventional leadframe having a narrow die pad, thesemiconductor device in accordance with the present aspect of theinvention has a smaller area in direct contact with molding resin. As aresult, there is less possibility of separation of the molding resinfrom the semiconductor chip. Thus, a higher-reliability semiconductordevice can be obtained.

In a leadframe in accordance with the sixth aspect of the presentinvention, the portions on both sides of an outer frame to which a diepad is connected are bent into the U-shape so as to sink the die pad.Compared to the case where the die pad is sunk by bending the suspendingleads on both sides of the die pad, this leadframe has advantages thatthe die pad can be sunk more deeply, and that larger mechanical strengthcan be obtained, and that it is possible to shorten the distance betweenleads extending from one side and those extending from the other side.The seventh aspect of the present invention provides a production methodwhereby a die pad extending perpendicular to the direction in which theleadframe is transferred during the processes of producing asemiconductor device is sunk by bending both side portions of the outerframe to which the die pad is connected into the U-shape, so that asemiconductor chip can be inserted from a location transverse to theleadframe-transfer path. Thus, the chip-insertion process can beperformed easily. Furthermore, when the side portions are bent into theU-shape so as to form the U-shaped portions, the die pad is sunk fromthe leads, and, at the same time, the leads extending from one sidebecome closer to the leads extending from the other side. As a result,it is possible to make the leads reach positions closer to the centralpart of the semiconductor chip, and it is possible to increaseflexibility in electrode location.

In accordance with the eighth and ninth aspects of the present inventionrelating to a leadframe and a method for producing a semiconductordevice using this leadframe wherein the leadframe includes a die pad andleads formed in an integral fashion, the die pad can be bent outwardfrom the outer frame, and in this situation where the die pad is bent, asemiconductor chip can be die-bonded onto this die pad. This allowseasier and more reliable processing, compared to the case where asemiconductor chip is inserted between a die pad and leads.

In accordance with the tenth and eleventh aspects of the presentinvention relating to a leadframe and a method for producing asemiconductor device using this leadframe wherein the unitary leadframeincludes a die pad and leads, the die pad is formed within an outerframe area adjacent to another outer frame area in which leads areformed, both sides of the die pad being connected to the outer frame viasuspending leads. A semiconductor chip is die-bonded onto the die pad,then an unnecessary portion of the outer frame around the die pad is cutoff. After that, the suspending lead connected to the side of the diepad closer to the lead area is bent into the U-shape such that the diepad having the mounted semiconductor chip is folded over the leads. Thisprocess can be performed easily. Furthermore, because the unnecessaryportion of the outer frame is removed, various advantages are obtainedsuch that the succeeding process steps are made easier, and thattransfer errors during the production processes can be decreased andproduction efficiency can be improved.

In accordance with the twelfth and thirteenth aspects of the presentinvention with regard to a leadframe and a method for producing asemiconductor device using this leadframe, more firm connection of thedie pad to an outer frame area for leads is achieved by modifying theleadframe and the method in accordance with the tenth and eleventhaspects of the invention in such a way that a suspending lead portionremaining connected to the die pad is bent and connected to the outerframe area for leads. Thus, the succeeding processes can be performedmore easily and more accurately.

In accordance with the fourteenth aspect of the present invention withregard to a die-bonding method, metal foil is placed on and pressed ontoa die pad, then a semiconductor chip is put on this metal foil and themetal foil is melted by heating it. Then, the heating is stopped so asto solidify the metal foil and to attach the semiconductor chip on thedie pad. This process step is carried out by shifting the leadframe byone pitch at a time. Thus, this method is suitable for use in aproduction line to die-bond a chip on a leadframe which is transferredon the production line. This method provides significant advantages inthat die-bonding process becomes easier and more effective andproduction efficiency can be improved, in particular where die-bondingis performed after a semiconductor chip is inserted between the die padand inner lead portions.

What is claimed is:
 1. A leadframe for producing a semiconductor devicehaving a lead-on-chip (LOC) structure wherein leads extend across asemiconductor chip comprising:a frame for a die pad comprising an outerframe section, a die pad displaced from said outer frame section, and asuspending lead connecting said die pad to said outer frame sectionwherein said die pad is disposed inside said outer frame section; and aframe for leads comprising an outer frame portion and a plurality ofleads extending from opposite sides of said outer frame portion andhaving at least one frame-cutting slit in said outer frame portionexposing said suspending lead for severing and removing an unnecessaryportion of said frame for a die pad after said frame for leads isconnected to said frame for a die pad, leaving said die pad connected tosaid frame for leads at said suspending lead, wherein one of said framefor a die pad and said frame for leads includes a projection and theother of said frame for a die pad and said frame for leads includes ahole, the hole receiving said projection, said projection being disposedparallel to said frame for leads, thereby connecting said frame for adie pad to said frame for leads.
 2. A semiconductor device having alead-on-chip (LOC) structure comprising:a semiconductor chip havingfirst and second primary surfaces and a plurality of electrodescentrally disposed on said first primary surface; a frame for a die padincluding a die pad and a suspending lead connected to said die pad,said semiconductor chip being bonded to said die pad at said secondprimary surface, a frame for leads comprising an outer frame portion anda plurality of leads extending from opposite sides of said outer frameportion, said frame for leads including at least one frame cutting slitin said outer frame portion providing access to said suspending lead forsevering and removing an unnecessary portion of said frame for a diepad, said frame for a die pad and said frame for leads being connectedat said outer frame portion so that said suspending lead is connected tosaid outer frame portion, said die pad is displaced from said outerframe portion, and inner lead portions of said leads extend across andare spaced from the first primary surface of said semiconductor chip,wherein one of said frame for a die pad and said frame for leadsincludes a projection and the other of said frame for a die pad and saidframe for leads includes a hole receiving the projection wherein theprojection is parallel to said outer frame portion; metal wires bondedto respective ones of said electrodes and corresponding ones of saidinner lead portions; and a resin encapsulating said semiconductor chip,said die pad, and said wires, said outer frame portion and said innerlead portions.